SCIENCE WATCH

SCIENCE WATCH; Horseshoe Crabs and Vision

Published: March 23, 1982

The horseshoe crab has long served as a subject for studies of the physiology of vision. But the role of vision in the animal's own behavior has seldom been investigated.

So three scientists from the Marine Biological Laboratory at Woods Hole, Mass., played some visual tricks on the animals to see how their mating behavior would be affected.

Every spring, horseshoe crabs move in to the beaches of Cape Cod to build nests. But there are more males than females, and the scientists designed various cement castings to serve surplus males as surrogate mating objects. There were three different shapes: a cube, a hemisphere, and one like an adult female's shell. And there were three different shades: black, gray and white.

These bogus mates were placed randomly below the high tide line and watched closely during eight high tides, four at night and four during the day, to see which ones the crabs would approach and try to mate with.

The males clearly preferred the casting shaped like a female horseshoe crab, and they liked the hemispheres better than the cubes. But whether they discriminated form through touch or vision or both was unclear.

However, other evidence suggested that vision was important. The males were attracted most often to the black castings and next most often to the gray, the two shades of paint that most resembled a female's shell.

The researchers conclude that vision plays a role in the horseshoe crab's mating behavior, though other senses may also be involved. Their report is in the March 4 issue of the journal Nature.